A Study On The Formability Of Knitted Reinforcing Structures

This paper mainly investigated the formability of knitted structures used for composite reinforcement. During advanced composites manufacturing process, in order to reduce the production cost, 2D fabrics need to be deformed directly on complex surfaces, which requests high deformabihty of fabrics. While the traditional woven fabrics have limited formability due to their shear deformation, the knitted fabrics have excellent formability due to their particular loop structures which can be easily deformed. Due to the importance of the formability in composites manufacturing, the stretching deformation characteristics and formability on complex surfaces for different knitted structures were experimentally investigated in this paper.Three types of glass fiber knitted fabrics, i.e., weft knitted, warp knitted, biaxial fabrics, were firstly chosen for the formability study. The knitting process and determination of knitting parameters for the production of the weft knitted fabrics on STOLL CMS303TC electronic flat knitting machine were discussed in detail. The structural features and parameter measurement of each fabric was also presented. From that, the knitted fabric samples were ready for the formability analysis.Because the formability is achieved by multi-axial stretching deformation of the knitted fabrics, the bursting tests were then carried out. The deformability of each knitted fabric and its relations with formability were investigated by analyzing the bursting curve at the initial stage. The testing results show that at a given bursting load, the stretching lengths of the weft knitted fabrics are bigger than those of the warp knitted fabrics, and the stretching lengths of the warp knitted fabrics are bigger than those of the biaxial knitted fabrics. The results also show that the deformability of 1×1 rib is best among all the weft knitted structures and that of the space warp knitted is best among all the warp knitted structures, and the weft biaxial knitted fabric has better deformability than the warp biaxial knitted fabric.Based on the bursting tests, the formability of all three kinds of the knitted fabrics were systematically analyzed and discussed. By using the method developed, the formability of the knitted fabric with different structures and loop lengths on the surfaces of the hemispheres, cylinders and cubes with different dimensions were experimentally investigated in detail. The method used is firstly splitting planar knitted fabrics into small grids by drawing lines, and then analyzing the variation of each grid. The conformable heights on cylinders and cubes and outlines after the fabrics were deformed to adapted different surfaces were given and compared. The variations of the areas before and after deformation as well as the fiber volume fraction changing trends were analyzed. The analyzing results show that the weft knitted fabrics can easily be deformed to adapt the hemispherical, cylindrical and cubic surfaces. The warp knitted fabrics can easily be deformed to adapt hemispherical and cylindrical surfaces, but they are difficult to be deformed to adapt the cubic surface with smaller dimensions. It is not possible to deform the biaxial knitted fabrics to adapt the cubic surface. The results also show that the big differences exist between the outline shapes of the weft knitted and those of the warp knitted fabrics, and that the differences also exist between different structures for the same kind of knitted fabrics.In order to demonstrate using good formability of the knitted fabrics to fabricate the composites with complex shape, a rib fabric produced with glass fiber yarns on an electronic flat knitting machine was chosen and directly deformed to adapt the surface of a boat-shaped mould. The boat composite shell was manufactured through RTM process. The method of laying-up the knitted fabric on the boat shaped mould and the whole manufacturing process were also presented.The results of this investigation have practical significance in promoting the application of the knitted structures for composite reinforcement.